When the evaporator coil of a refrigeration system is operating at or near full load, the evaporator coil is almost fully flooded with refrigerant. When the evaporator coil is almost fully flooded, the temperature of the coil across its length will be very uniform, and thus air flowing across the evaporator coil will have a uniform discharge temperature across the coil length. This is very important in transport refrigeration systems, as perishables have a shelf life dependent upon the ability of the transport refrigeration system to maintain a desired set point temperature. Only a few degrees temperature difference may deleteriously affect the shelf life of a perishable product in the cargo space of a truck, trailer, container, and the like.
In an effort to maintain the temperature of the served cargo space as closely as possible to set point, and thus obtain the shelf life advantage, suction line modulation is being increasingly used by refrigeration system control algorithms to reduce the mass flow of refrigerant when the sensed temperature is close to the predetermined set point temperature For example, U.S. Pat. No. 4,899,549, which is assigned to the same assignee as the present application, discloses a transport refrigeration system which has a suction line modulation valve, with the associated refrigeration control providing suction line modulation in cooling and heating cycles above and below set point, respectively.
While suction line modulation enables a sensed temperature to be held closer to set point, controlling the cooling capacity of a refrigeration system by reducing the refrigerant mass flow may result in only a small portion of the evaporator coil being flooded with refrigerant when extensive capacity reduction is required. As a result, the air temperature along the length of the evaporator coil may not be uniform, i.e., the evaporator coil will be colder at the refrigerant distribution end of the evaporator coil than at the opposite end.
Accordingly, it would be desirable, and it is an object of the invention, to be able to provide a more uniform temperature of air flow across, i.e., transverse to, the length dimension of an evaporator coil, especially with refrigeration systems which may only partially flood an evaporator coil with refrigerant during their operation, such as those which utilize suction line modulation to reduce cooling and heating capacity near set point.